Stabilization agents for silver nanowire based transparent conductive films

ABSTRACT

Zinc salts have been found to provide anticorrosion properties when incorporated into silver nanowire containing films. Such salts may be incorporated into one of more silver nanowire containing layers or in one or more layers disposed adjacent to the silver nanowire containing layers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/778,555, filed Mar. 13, 2013, entitled STABILIZATION AGENTS FOR SILVER NANOWIRE BASED TRANSPARENT CONDUCTIVE FILMS, which is hereby incorporated by reference in its entirety.

BACKGROUND

Transparent and conductive films (TCF) have been used extensively in recent years in applications such as touch panel displays, liquid crystal displays, electroluminescent lighting, organic light-emitting diode devices, and photovoltaic solar cells. Indium tin oxide (ITO) based transparent conductive film has been the transparent conductor-of-choice for most applications due to its high conductivity, transparency, and relatively good stability. However, indium tin oxide based transparent conductive films have limitations due to the high cost of indium, the need for complicated and expensive vacuum deposition equipment and processes, and indium tin oxide's inherent brittleness and tendency to crack, especially when it is deposited on flexible substrates.

Two of the most important parameters for measuring the properties of transparent conductive films are total light transmittance (% T) and film surface electric conductivity. Higher light transmittance allows clear picture quality for display applications, higher efficiency for lighting and solar energy conversion applications. Lower resistivity (R) is most desirable for most transparent conductive films applications in which power consumption can be minimized. Therefore, the higher the T/R ratio of the transparent conductive films is, the better the transparent conductive films are.

U.S. Patent Application Publication 2006/0257638A1 discloses a transparent conductive film comprising carbon nanotubes (CNT) and vinyl chloride resin polymer binder.

U.S. Pat. No. 8,049,333 and U.S. Patent Application Publication 2008/0286447A1 disclose a transparent conductive film in which silver nanowires are deposited onto a substrate to form a bare nanowire network followed by overcoating the silver nanowire network with a polymer matrix material to form a transparent conductive film. Polymers such as polyacrylates and carboxyl alkyl cellulose ether polymers were suggested as useful materials for the matrix.

US Patent Application Publication 2008/0286447A1 discloses the use of aromatic triazoles and other nitrogen containing compounds as corrosion inhibitors for silver nanowire based transparent conductors. Long chain alkylthio compounds have also been disclosed as useful corrosion inhibitors.

U.S. Patent Application Publication 2008/0292979A1 discloses a transparent conductive film comprising silver nanowires, or a mixture of silver nanowires and carbon nanotubes. The transparent conductive network is formed either without polymer binder or in a photoimageable composition. The transparent conductive films were coated on both glass and polyethylene terephthalate (PET) supports.

U.S. Pat. No. 8,052,773 describes a transparent conductive film which is formed from coating of silver nanowires to form a network followed by overcoating with a layer of urethane acrylate polymer.

U.S. Patent Application Publication 2011/0024159A1 discloses use of corrosion inhibitors in an overcoat layer of a transparent conductive film.

PCT Patent Publication WO 2011/115603 discloses anticorrosion agents comprising 1,2-diazine compounds for use in transparent conductive films.

US Patent Application Publication 2010/0307792A1 discloses addition of coordination ligands with silver nanowire aqueous dispersions to form sediments followed by separation of such sediments from the supernatant containing halide ions before applying such silver nanowire dispersions in the coating and formation of TCF.

European Patent EP2251389B1 discloses a silver nanowire based ink formulation in which various aqueous silver complex ions were added into silver nanowire based ink in a ratio of complex ion to silver nanowire of no more than 1:64 (w:w).

SUMMARY OF THE INVENTION

Zinc salts are particularly useful as anticorrosion agents for the stabilization of a network of silver nanowire-based transparent conductive films toward the undesirable reaction of such conductive films with corrosive agents such as hydrogen sulfide.

We have discovered that the effectiveness of zinc salts may be enhanced by their introduction in at least one coating mix for at least one layer disposed adjacent to the at least one layer comprising silver nanowires. Such a layer might be an overcoat or topcoat layer, if disposed on the at least one layer comprising silver nanowires. Such an overcoat or topcoat layer may, for example, be thermally cured or UV cured. Alternatively, such a layer might be a primer or undercoat layer, if disposed between the at least one layer comprising silver nanowires and the transparent support. Or the zinc salt might be included in layers both above and below the at least one layer comprising silver nanowires. In any of these cases, the zinc salt may, optionally, also be added to at least one of the layers comprising silver nanowires.

At least a first embodiment provides a transparent conductive article comprising a transparent support; at least one first layer disposed on the transparent support, the at least one first layer comprising a network of silver nanowires dispersed within a polymer binder; and at least one second layer disposed on the at least one first layer, the at least one second layer comprising at least one zinc salt defined as any chemical substance which, when dissolved or put in a molten state, produces zinc cation.

In at least some embodiments, the at least one second layer may further comprise at least one organic acid.

In at least some such embodiments, the at least one first layer may further comprise at least one zinc salt.

In yet further embodiments, the at least one first layer may comprise at least one zinc salt, as defined above, and further comprise at least one organic acid.

At least a second embodiment provides a transparent conductive article comprising a transparent support; at least one first layer disposed on the transparent support, the at least one first layer comprising at least one zinc salt; and at least one second layer disposed on the at least one first layer, the at least one second layer comprising a network of silver nanowires dispersed within a polymer binder.

In at least some such embodiments, the at least one first layer may further comprise at least one organic acid.

In at least some such embodiments, the at least one second layer may further comprise at least one zinc salt.

In yet further embodiments, the at least one second layer may comprise at least one zinc salt, and further comprise at least one organic acid.

At least a third embodiment provides a transparent conductive article comprising a transparent support; at least one first layer disposed on the transparent support; at least one second layer disposed on the at least one first layer, the at least one second layer comprising a network of silver nanowires dispersed within a polymer binder; at least one third layer disposed on the at least one second layer, the at least one third layer comprising at least one zinc salt.

In at least some embodiments, the at least one third layer may further comprise at least one organic acid.

In at least some such embodiments, the at least one second layer may further comprise at least one zinc salt.

In yet further embodiments, the at least one second layer may comprise at least one zinc salt, as defined above, and further comprise at least one organic acid.

At least a fourth embodiment provides methods comprising applying at least one first coating mixture onto a transparent support to form at least one first coated layer, the at least one first coating mixture comprising silver nanowires and at least one polymer binder; and applying at least one second coating mixture onto the at least one first coated layer to form at least one second coated layer, the at least one second coating mixture comprising at least one zinc salt.

In at least some such embodiments, the at least one second coating mixture may further comprise at least one organic acid.

In at least some such embodiments, the at least one first coating mixture may further comprise at least one zinc salt.

In yet further embodiments, the at least one first coating mixture may comprise at least one zinc salt and at least one organic acid

At least a fifth embodiment provides methods comprising applying at least one first coating mixture onto a transparent support to form at least one first coated layer, the at least one first coating mixture comprising at least one zinc salt, and applying at least one second coating mixture onto the at least one first coated layer, where the at least one second coating mixture comprises silver nanowires and at least one polymer binder.

In at least some such embodiments, the at least one first coating mixture may further comprise at least one organic acid.

In at least some such embodiments, the at least one second coating mixture may further comprise at least one zinc salt.

In further such embodiments, the at least one second coating mixture may comprise at least one zinc salt and at least one organic acid.

In any of the above embodiments, the organic acid may, in some cases, comprise at least one of maleic acid (MA), tetrachlorophthalic acid (TCPA), trichloroacetic acid (TCAA), phenylphosphonic acid (PPOA), p-toluenesulfonic acid (PTSA), and phthalic acid (PA); or, in other cases, the organic acid may comprise at least one of maleic acid (MA) or p-toluenesulfonic acid (PTSA); or, in still other cases, the organic acid may comprise maleic acid. The organic acid may, for example, have a pKa value from about −3.5 to about 3.0, or from about 1.85 to about 3.0.

DESCRIPTION

All publications, patents, and patent documents referred to in this document are incorporated by reference in their entirety, as though individually incorporated by reference.

U.S. Provisional Application No. 61/778,555, filed Mar. 13, 2013, entitled STABILIZATION AGENTS FOR SILVER NANOWIRE BASED TRANSPARENT CONDUCTIVE FILMS, is hereby incorporated by reference in its entirety.

DEFINITIONS

The terms “conductive layer” or “conductive film” refer to the network layer comprising silver nanowires dispersed within a polymer binder.

The term “conductive” refers to electrical conductivity.

The term “article” refers to the coating of a “conductive layer” or “conductive film” on a support.

The terms “coating weight,” “coat weight,” and “coverage” are synonymous, and are usually expressed in weight or moles per unit area such as g/m² or mol/m².

The term “transparent” means capable of transmitting visible light without appreciable scattering or absorption.

“Haze” is wide-angle scattering that diffuses light uniformly in all directions. It is the percentage of transmitted light that deviates from the incident beam by more than 2.5 degrees on the average. Haze reduces contrast and results in a milky or cloudy appearance. Materials having lower haze percentages appear less hazy than those having higher haze percentages.

The term “organic solvent” means “a material, liquid at use temperature, whose chemical formula comprises one or more carbon atoms.”

The term “aqueous solvent” means a material, liquid at use temperature, whose composition in a homogeneous solution comprises water in the greatest proportion (i.e., at least 50 percent water by weight).

The term “water soluble” means the solute forms a homogenous solution with water, or a solvent mixture in which water is the major component.

The terms “a” or “an” refer to “at least one” of that component (for example, the anticorrosion agents, nanowires, and polymers described herein).

Furthermore, all publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference.

Introduction

In order for silver based transparent conductors to have practical use it is important that these silver based transparent conductors be stable for a long period when subjected to environmental conditions.

Any atmospheric corrosion due to the reaction of low levels of chemicals in the air may induce undesirable chemical reactions at the metal nanowire surface, impacting the conductivity and performance of the metal nanowire based transparent conductors. It is well known that corrosion, or “tarnishing,” may readily occur on silver metal surfaces when exposed to the atmosphere. Without wishing to be bound by theory, one example of such a tarnishing mechanism is sulfidation of silver surface by reaction of hydrogen sulfide with silver: 2Ag+H₂S→Ag₂S+H₂

Because the electric conductivity of silver compounds such as silver sulfide is much lower than that of silver metal, silver nanowire based conductors can gradually lose conductivity when exposed to the atmosphere.

In contrast to bare metal wires exposed to the air, silver nanowires in a polymer matrix are more stable since the presence of the polymer slows down the diffusion of hydrogen sulfide (or other corrosive agents) to the silver nanowire surface. Nevertheless, it is important to stabilize the silver nanowire surface to prevent the sulfidation process, even when the nanowires are embedded in a polymer matrix.

It would be useful to find anticorrosion agents for transparent electrically conductive films comprising a network of silver nanowires in polymer binder(s) that can be coated from aqueous or from organic solvents, using common coating techniques.

Silver Nanowires

The silver nanowires are an essential component for imparting electrical conductivity to the conductive films, and to the articles prepared using the conductive films. The electrical conductivity of the transparent conductive film is mainly controlled by a) the conductivity of a single nanowire, b) the number of nanowires between the terminals, and c) the number of connections and the contact resistivity between the nanowires. Below a certain nanowire concentration (also referred as the percolation threshold), the conductivity between the terminals is zero, as there is no continuous current path provided because the nanowires are spaced too far apart. Above this concentration, there is at least one current path available. As more current paths are provided, the overall resistance of the layer will decrease. However, as more current paths are provided, the clarity (i.e., percent light transmission) of the conductive film decreases due to light absorption and back scattering by the nanowires. Also, as the amount of silver nanowires in the conductive film increases, the haze of the transparent film increases due to light scattering by the silver nanowires. Similar effects will occur in transparent articles prepared using the conductive films.

In one embodiment, the silver nanowires have aspect ratio (length/width) of from about 20 to about 3300. In another embodiment, the silver nanowires have an aspect ratio (length/width) of from about 500 to 1000. Silver nanowires having a length of from about 5 μm to about 100 μm (micrometer) and a width of from about 10 nm to about 200 nm are useful. Silver nanowires having a width of from about 20 nm to about 100 nm and a length of from about 10 μm to about 50 μm are also particularly useful for construction of a transparent conductive network film.

Silver nanowires can be prepared by known methods in the art. In particular, silver nanowires can be synthesized through solution-phase reduction of a silver salt (e.g., silver nitrate) in the presence of a polyol (e.g., ethylene glycol or propylene glycol) and poly(vinyl pyrrolidone). Large-scale production of silver nanowires of uniform size can be prepared according to the methods described in, e.g., Ducamp-Sanguesa, C. et al., J. of Solid State Chemistry, (1992), 100, 272-280; Sun, Y. et al., Chem. Mater. (2002), 14, 4736-4745, Sun, Y. et al., Nano Letters, (2003), 3(7), 955-960; US patent application publication 2012/0063948, published Mar. 15, 2012; US patent application publication 2012/0126181, published May 24, 2012; US patent application publication 2012/0148436, published Jun. 14, 2012; US patent application publication 2012/0207644, published Aug. 16, 2012; and US patent application publication 2012/0328469, published Dec. 27, 2012, each of which is incorporated by reference in its entirety.

Polymer Binders

For a practical manufacturing process for transparent conductive films, it is important to have both the conductive components, such as silver nanowires, and a polymer binder in a coating dispersion. The polymer binder solution serves a dual role, as dispersant to facilitate the dispersion of silver nanowires and as a viscosifier to stabilize the silver nanowire coating dispersion so that the sedimentation of silver nanowires does not occur at any point during the coating process. It is also desirable to have the silver nanowires and the polymer binder in a single coating dispersion. This simplifies the coating process and allows for a one-pass coating, and avoids the method of first coating bare silver nanowires to form a weak and fragile film that is subsequently over-coated with a polymer to form the transparent conductive film.

In order for a transparent conductive film to be useful in various device applications, it is also important for the polymer binder of the transparent conductive film to be optically transparent and flexible, yet have high mechanical strength, good hardness, high thermal stability, and light stability. This requires polymer binders to be used for transparent conductive film to have Tg (glass transition temperature) greater than the use temperature of the transparent conductive film.

Transparent, optically clear polymer binders are known in the art. Examples of suitable polymeric binders include, but are not limited to: polyacrylics such as polymethacrylates (e.g., poly(methyl methacrylate)), polyacrylates and polyacrylonitriles, polyvinyl alcohols, polyesters (e.g., polyethylene terephthalate (PET), polybutylene terephthalate, and polyethylene naphthalate), polymers with a high degree of aromaticity such as phenolics or cresol-formaldehyde (e.g., NOVOLAC®), polystyrenes, polyvinyltoluene, polyvinylxylene, polyimides, polyamides, polyamideimides, polyetheramides, polysulfides, polysulfones, polyphenylenes, and polyphenyl ethers, polyurethane (PU), polycarbonates, epoxies polyolefins (e.g. polypropylene, polymethylpentene, and cyclic olefins), acrylonitrile-butadiene-styrene copolymers (ABS), cellulosics, silicones and other silicon-containing polymers (e.g. polysilsesquioxanes and polysilanes), polyvinylchloride (PVC), polyvinylacetates, polynorbornenes, synthetic rubbers (e.g. EPR, SBR, EPDM), and fluoropolymers (e.g., polyvinylidene fluoride, polytetrafluoroethylene (TFE) or polyhexafluoropropylene), copolymers of fluoro-olefins and hydrocarbon olefins (e.g., LUMIFLON®), and amorphous fluorocarbon polymers or copolymers (e.g., CYTOP® by Asahi Glass Co., or TEFLON® AF by Du Pont), polyvinylbutryals, polyvinylacetals, gelatins, polysaccharides, and starches.

In certain embodiments, in order to disperse and stabilize silver nanowires in polymeric coating solution, the use of polymer binders having high oxygen content is advantageous. Oxygen-containing groups, such as hydroxyl group and carboxylate groups have a strong affinity for binding to the silver nanowire surface and facilitate the dispersion and stabilization. Many oxygen-rich polymers also have good solubility in the polar organic solvents commonly used to prepare organic solvent-coated materials, while other oxygen-rich polymers have good solubility in water or the aqueous solvent mixtures commonly used to prepare aqueous solvent-coated materials.

In certain embodiments, cellulose ester polymers, such as cellulose acetate butyrate (CAB), cellulose acetate (CA), or cellulose acetate propionate (CAP) are superior to other oxygen-rich polymer binders when used to prepare silver nanowire based transparent conductive films that are coated from organic solvents such as 2-butanone (methyl ethyl ketone, MEK), methyl iso-butyl ketone, acetone, methanol, ethanol, 2-propanol, ethyl acetate, propyl acetate, butyl acetate, or mixtures thereof. Their use results in transparent conductive films in which both the optical light transmittance and electrical conductivity of the coated films are greatly improved. In addition, these cellulose ester polymers have glass transition temperatures of at least 100° C. and provide transparent, flexible films having high mechanical strength, good hardness, high thermal stability, and light stability.

The cellulose ester polymers can be present in from about 40 to about 90 wt % of the dried transparent conductive films. Preferably, they are present in from about 60 to about 85 wt % of the dried films. In some constructions, a mixture of a cellulosic ester polymer and one or more additional polymers may be used. These polymers should be compatible with the cellulosic polymer. By compatible is meant that a mixture comprising at least one cellulosic ester polymer and one or more additional polymers forms a transparent, single phase composition when dried. The additional polymer or polymers can provide further benefits such as promoting adhesion to the support and improving hardness and scratch resistance. As above, total wt % of all polymers is from about 40 to about 95 wt % of the dried transparent conductive films. Preferably, the total weight of all polymers is from about 60 to about 85 wt % of the dried films. Polyester polymers, urethanes, and polyacrylics are examples of additional polymers useful for blending with cellulosic ester polymers.

In other embodiments, water soluble polymer binders can also be used, such as polyvinyl alcohol, gelatin, polyacrylic acid, polyimides. Other water dispersible latex polymers can also be used such as polyacrylates and polymethacrylates containing methyl acrylic acid units. Coating from aqueous solutions benefits the environment and reduces the emission of volatile organic compounds during manufacturing.

The use of water soluble polymers, such as polyvinyl alcohol or gelatin as binders for silver nanowire based transparent conductors results in superior transparent conductive films in which both film transmittance and conductivity are greatly improved. Transparent conductive films prepared using either polyvinyl alcohol or gelatin polymer binders also show excellent clarity, scratch resistance, and hardness when polymer cross linkers are added to the polymer solution. Transparent conductive films prepared according to this invention provide transmittance of at least 80% across entire spectrum range of about 350 nm to about 1100 nm, and surface resistivity of 500 ohm/sq or less.

The transparent conductive articles comprising silver nanowires and water soluble polymer binders also show excellent clarity, high scratch resistance, and hardness. In addition, transparent conductive films prepared using these polymer binders have good adhesion to supports comprising polyethylene terephthalate (PET), poly(methylmethacrylate), polycarbonate, and the like, when an appropriate subbing layer is applied between the support and the conductive layer.

The water soluble polymer binders are present in from about 40 to about 95 wt % of the dried transparent conductive films. Preferably, they are present in from about 60 to about 85 wt % of the dried films.

In some constructions, up to 50 wt % of the gelatin or polyvinyl alcohol polymer binder can be replaced by one or more additional polymers. These polymers should be compatible with the gelatin or polyvinyl alcohol polymer binder. By compatible is meant that the all polymers form a transparent, single phase mixture when dried. The additional polymer or polymers can provide further benefits such as promoting adhesion to the support and improving hardness and scratch resistance. Water soluble acrylic polymers are particularly preferred as additional polymers. Examples of such polymers are polyacrylic acid and polyacrylamides, and copolymers thereof. As above, total wt % of all polymers is from about 50 to about 95 wt % of the dried transparent conductive films. Preferably, the total weight of all polymers is from about 70 to about 85 wt % of the dried films.

If desired, scratch resistance and hardness of the transparent conductive films with these polymer binders to the support can be improved by use of crosslinking agents to crosslink the polymer binders. Isocyanates, alkoxyl silanes, and melamines are examples of typical crosslinking agents for cellulose ester polymers containing free hydroxyl groups. Vinyl sulfones and aldehydes are examples of typical crosslinking agents for gelatin binders.

Stabilization Agents

Stabilization agents are chemical compounds that, when added to the transparent conductive film, improve the stability of the construction with respect to atmospheric corrosion caused by the reaction of oxygen, or one or more other chemicals in the atmosphere, with one or more components in the film. This reaction results in deterioration of the electric conductivity, optical properties, and/or physical integrity of the film. Stabilization agents should be colorless and odorless when used in the transparent conductive film, and should be stable to the conditions of heat, light, and humidity in the environment where transparent conductive film is used.

However, in practice, many such compounds, when bound to a silver nanowire surface, will drastically reduce the electric conductivity of the resultant conductive film. Apparently, the insulating effect of these compounds prevents electron “flow” at nanowire contact points. Therefore, it is important to identify a class of compounds that will provide anticorrosion protection to transparent conductive film without causing significant reduction in conductivity and other negative effects. Advantageously, delaying introduction of the anticorrosion agents into the conductive nanowire network until after its formation can minimize the destruction of conductive paths in the network.

We have found that zinc salts have anti-corrosive and stabilizing effects when incorporated into silver nanowire containing films. Without wishing to be bound by theory, Applicants believe that zinc salts can be effective sulfide ion scavengers since Zn²⁺ cations can react with sulfide ions to form insoluble ZnS salts, thereby greatly reducing the actual concentration of hydrogen sulfide available to react with silver nanowires after diffusing from air into AgTCF layer.

In some embodiments, the zinc salts may comprise zinc nitrate hydrate (Zn(NO₃)₂.xH₂O). Such a hydrate of zinc nitrate may be provided as one of its particular hydrates or as a mixture of one or more of its hydrates. Zinc nitrate is known to have hydrates for integral values of x from 2 to 6.

In at least some embodiments, anticorrosion compounds may comprise at least one zinc salt and further comprise at least one organic acid. Some non-limiting examples of organic acids are maleic acid (MA), tetrachlorophthalic acid (TCPA), trichloroacetic acid (TCAA), phenylphosphonic acid (PPOA), p-toluenesulfonic acid (PTSA), and phthalic acid (PA). In at least some embodiments, the organic acids comprise pKa values from about −3.5 to about 3.0.

In yet further embodiments the anticorrosion compound may comprise zinc nitrate hydrate (Zn(NO₃)₂.xH₂O) and at least one of the following six organic acids:

In at least some embodiments, the organic acids have pKa values from about −3.5 to about 3.0, or from about 1.85 to about 3.0.

In some cases, the organic acid comprises at least one of maleic acid or para-toluenesulfonic acid. In still other cases, the organic acid comprises maleic acid.

Coating of the Conductive Films

An organic solvent-based coating formulation for the transparent silver nanowire films can be prepared by mixing the various components with one or more polymer binders in a suitable organic solvent system that usually includes one or more solvents such as toluene, 2-butanone (methyl ethyl ketone, MEK), methyl iso-butyl ketone, acetone, methanol, ethanol, 2-propanol, ethyl acetate, propyl acetate, butyl acetate, ethyl lactate, tetrahydrofuran, or mixtures thereof. An aqueous-based coating formulation for the transparent silver nanowire films can be prepared by mixing the various components with one or more polymer binders in water or in a mixture of water with a water miscible solvent such as acetone, acetonitrile, methanol, ethanol, 2-propanol, or tetrahydrofuran, or mixtures thereof. Transparent films containing silver nanowires can be prepared by coating the formulations using various coating procedures such as wire wound rod coating, dip coating, knife, or blade coating, curtain coating, slide coating, slot-die coating, roll coating, or gravure coating. Surfactants and other coating aids can be incorporated into the coating formulation.

In one embodiment the coating weight of the silver nanowires is from about 10 mg/m² to about 500 mg/m². In another embodiment the coating weight of silver nanowires is from about 20 m g/m² to about 200 mg/m². In a further embodiment, the coating weight of silver nanowires is from about 30 mg/m² to about 120 mg/m². A useful coating dry thickness of the transparent conductive coating is from about 0.05 μm to about 2.0 μm, and preferably from about 0.1 μm to about 0.5 μm.

Upon coating and drying, the transparent conductive film should have a surface resistivity of less than 1,000 ohms/sq and preferably less than 500 ohm/sq.

Upon coating, and drying, the transparent conductive film should have as high a % transmittance as possible. A transmittance of at least 70% is useful. A transmittance of at least 80% and even at least 90% are even more useful.

Particularly useful are films with a transmittance of at least 70% and a surface resistivity of less than 500 ohm/sq.

Such transparent conductive films provide transmittance of at least 80% across entire spectrum range of from about 350 nm to about 1100 nm, and surface resistivity of less than 500 ohm/sq.

Transparent Support

In one embodiment, the conductive materials are coated onto a support. The support may be rigid or flexible.

Suitable rigid substrates include, for example, glass, polycarbonates, acrylics, and the like.

When the conductive materials are coated onto a flexible support, the support is preferably a flexible, transparent polymeric film that has any desired thickness and is composed of one or more polymeric materials. The support is required to exhibit dimensional stability during coating and drying of the conductive layer and to have suitable adhesive properties with overlying layers. Useful polymeric materials for making such supports include polyesters [such as poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN)], cellulose acetates and other cellulose esters, polyvinyl acetal, polyolefins, polycarbonates, and polystyrenes. Preferred supports are composed of polymers having good heat stability, such as polyesters and polycarbonates. Support materials may also be treated or annealed to reduce shrinkage and promote dimensional stability. Transparent multilayer supports can also be used.

Coating of the Conductive Films onto a Support

Transparent conductive articles can be prepared by coating the formulations described above onto a transparent support using various coating procedures such as wire wound rod coating, dip coating, knife coating, curtain coating, slide coating, slot-die coating, roll coating, gravure coating, or extrusion coating.

Alternatively, transparent conductive articles can be prepared by laminating the transparent conductive films prepared as described above onto a transparent support.

In some embodiments, a “carrier” layer formulation comprising a single-phase mixture of two or more polymers may be applied directly onto the support and thereby located between the support and the silver nanowire layer. The carrier layer serves to promote adhesion of the support to the transparent polymer layer containing the silver nanowires. The carrier layer formulation can be sequentially or simultaneously applied with application of the transparent conductive silver nanowire layer formulation. It is preferred that all coating be applied simultaneously onto the support. Carrier layers are often referred to as “adhesion promoting layers,” “interlayers,” or “intermediate layers.”

As noted above, in one embodiment the coating weight of the silver nanowires is from about 20 mg/m² to about 500 mg/m². In other embodiments, coating weight of silver nanowires is from about 10 m g/m² to about 200 mg/m². Embodiments wherein the silver nanowires are coated at from about 10 mg/m² to about 120 mg/m² are also contemplated.

Upon coating and drying, the transparent conductive article should have a surface resistivity of less than 1,000 ohms/sq and preferably less than 500 ohm/sq.

Similarly, upon coating and drying on a transparent support, the transparent conductive article should have as high an optical transmittance as possible. A transmittance of at least 70% is useful. A transmittance of at least 80% and even at least 90% are even more useful.

Particularly preferred are articles with a transmittance of at least 80% and a surface resistivity of less than 500 ohm/sq.

The following examples are provided to illustrate the practice of the present invention and the invention is not meant to be limited thereby.

ENUMERATED EMBODIMENTS

U.S. Provisional Application No. 61/778,555, filed Mar. 13, 2013, entitled STABILIZATION AGENTS FOR SILVER NANOWIRE BASED TRANSPARENT CONDUCTIVE FILMS, which is hereby incorporated by reference in its entirety, disclosed the following 58 non-limiting enumerated embodiments:

A. A transparent conductive article comprising:

a transparent support;

at least one first layer disposed on the transparent support, the at least one first layer comprising a network of silver nanowires dispersed within at least one polymer binder, and;

at least one second layer disposed on the at least one first layer, the at least one second layer comprising at least one zinc salt.

B. The transparent conductive article of embodiment A, wherein the at least one zinc salt comprises zinc nitrate hydrate.

C. The transparent conductive article of embodiment A, wherein the at least one second layer further comprises at least one organic acid.

D. The transparent conductive article of embodiment C, wherein the at least one organic acid comprises at least one of maleic acid, tetrachlorophthalic acid, trichloroacetic acid, phenylphosphonic acid, p-toluenesulfonic acid, or phthalic acid.

E. The transparent conductive article according to embodiment C, wherein the at least one organic acid comprises at least one of maleic acid or p-toluenesulfonic acid.

F. The transparent conductive article according to embodiment C, wherein the at least one organic acid comprises maleic acid.

G. The transparent conductive article of embodiment A, wherein the transparent support is a flexible transparent polymer film.

H. The transparent conductive article of embodiment A, wherein the silver nanowires are present in an amount sufficient to provide a surface resistivity of less than about 1000 ohm/sq.

J. The transparent conductive article of embodiment A, wherein the silver nanowires have an aspect ratio of from about 20 to about 3300.

K. The transparent conductive article of embodiment A, wherein the silver nanowires are present in an amount of from about 10 mg/m² to about 500 mg/m².

L. The transparent conductive article of embodiment A, having a transmittance of at least about 80% across entire spectrum range of from about 350 nm to about 1100 nm and a surface resistivity of 500 ohm/sq or less.

M. The transparent conductive article of embodiment A, wherein the at least one polymer binder comprises at least one water soluble polymer.

N. The transparent conductive article of embodiment M, wherein the at least one water soluble polymer comprises gelatin, polyvinyl alcohol, or mixtures thereof.

P. The transparent conductive article of embodiment N, wherein the at least one polymer binder further comprises up to about 50 wt % of one or more additional water soluble polymers.

Q. The transparent conductive article of embodiment P, wherein one or more of the additional water soluble polymers is a polyacrylic polymer.

R. The transparent conductive article of embodiment A, wherein the at least one polymer binder comprises an organic solvent soluble polymer.

S. The transparent conductive article of embodiment R, wherein the organic solvent soluble polymer binder comprises at least one cellulose ester polymer.

T. The transparent conductive article of embodiment R, wherein the organic solvent soluble polymer binder comprises cellulose acetate, cellulose acetate butyrate, or cellulose acetate propionate, or mixtures thereof.

U. The transparent conductive article of embodiment S, wherein the at least one cellulose ester polymer has a glass transition temperature of at least about 100° C.

V. The transparent conductive article of embodiment R, wherein the at least one polymer binder further comprises up to 50 wt % of one or more additional organic solvent soluble polymers.

W. The transparent conductive article of embodiment V, wherein one or more of the additional organic solvent soluble polymers is a polyester polymer.

X. A transparent conductive article comprising:

a transparent support;

at least one first layer disposed on the transparent support, the at least one first layer comprising a network of silver nanowires and a polymer binder, and at least one zinc salt; and,

at least one second layer consisting of a transparent polymer.

Y. The transparent conductive article of embodiment X, wherein the said at least one zinc salt comprises zinc nitrate hydrate.

Z. The transparent conductive article of embodiment X, wherein the at least one first layer further comprises at least one organic acid.

AA. The transparent conductive article of embodiment Z, wherein the at least one organic acid comprises at least one of maleic acid, tetrachlorophthalic acid, trichloroacetic acid, phenylphosphonic acid, p-toluenesulfonic acid, or phthalic acid.

AB. The transparent conductive article according to embodiment Z, wherein the at least one organic acid comprises at least one of maleic acid or p-toluenesulfonic acid.

AC. The transparent conductive article according to embodiment Z, wherein the organic acid comprises maleic acid.

AD. The transparent conductive article of embodiment X, wherein the transparent support is a flexible transparent polymer film.

AE. The transparent conductive article of embodiment X, wherein the silver nanowires are present in an amount sufficient to provide a surface resistivity of less than about 1000 ohm/sq.

AF. The transparent conductive article of embodiment X, wherein the silver nanowires have an aspect ratio of from about 20 to about 3300.

AG. The transparent conductive article of embodiment X, wherein the silver nanowires are present in an amount of from about 10 mg/m² to about 500 mg/m².

AH. The transparent conductive article of embodiment X, having a transmittance of at least about 80% across entire spectrum range of from about 350 nm to about 1100 nm and a surface resistivity of 500 ohm/sq or less.

AJ. The transparent conductive article of embodiment X, wherein the at least one polymer binder comprises at least one water soluble polymer.

AK. The transparent conductive article of embodiment AJ, wherein the at least one water soluble polymer comprises gelatin, polyvinyl alcohol, or mixtures thereof.

AL. The transparent conductive article of embodiment AK, wherein the at least one polymer binder further comprises up to about 50 wt % of one or more additional water soluble polymers.

AM. The transparent conductive article of embodiment AL, wherein one or more of the additional water soluble polymers is a polyacrylic polymer.

AN. The transparent conductive article of embodiment X, wherein the at least one polymer binder comprises an organic solvent soluble polymer.

AP. The transparent conductive article of embodiment AN, wherein the organic solvent soluble polymer binder comprises at least one cellulose ester polymer.

AQ. The transparent conductive article of embodiment AP, wherein the at least one cellulose ester polymer has a glass transition temperature of at least about 100° C.

AR. The transparent conductive article of embodiment AN, wherein the organic solvent soluble polymer binder comprises cellulose acetate, cellulose acetate butyrate, or cellulose acetate propionate, or mixtures thereof.

AS. The transparent conductive article of embodiment X, wherein the at least one polymer binder further comprises up to 50 wt % of one or more additional organic solvent soluble polymers.

AT. The transparent conductive article of embodiment AS, wherein one or more of the additional organic solvent soluble polymers is a polyester polymer.

AU. A method comprising:

applying at least one first coating mixture onto a transparent support to form at least one first coated layer, the at least one first coating mixture comprising silver nanowires and at least one polymer binder; and,

applying at least one second coating mixture onto the at least one first coated layer to form at least one second coated layer, the at least one second coating mixture comprising at least one zinc salt.

AV. The method according to embodiment AU, wherein the at least one zinc salt comprises zinc nitrate hydrate.

AW. The method according to embodiment AU, wherein the at least one second coating mixture further comprises at least one organic acid.

AX. The transparent conductive article of embodiment AW, wherein the at least one organic acid comprises at least one of maleic acid, tetrachlorophthalic acid, trichloroacetic acid, phenylphosphonic acid, p-toluenesulfonic acid, or phthalic acid.

AY. The method according to embodiment AW, wherein the at least one organic acid comprises at least one of maleic acid or p-toluenesulfonic acid.

AZ. The method according to embodiment AW, wherein the at least one organic acid comprises maleic acid.

BA. The method according to embodiment AU, wherein applying the at least one first coating mixture and applying the at least one second coating mixture occur simultaneously.

BB. The method according to embodiment AU, further comprising drying the at least one first layer or the at least one second layer or both.

BC. A method comprising:

applying at least one first coating mixture onto a transparent support to form at least one first coated layer, the at least one first coating mixture comprising at least one zinc salt, silver nanowires, and at least one polymer binder; and,

applying at least one second coating mixture onto the at least one first coated layer.

BD. The method according to embodiment BC, wherein the at least one zinc salt comprises zinc nitrate hydrate.

BE. The method according to embodiment BC, wherein the at least one first coating mixture further comprises at least one organic acid.

BF. The transparent conductive article of embodiment BE, wherein the at least one organic acid comprises at least one of maleic acid, tetrachlorophthalic acid, trichloroacetic acid, phenylphosphonic acid, p-toluenesulfonic acid, or phthalic acid.

BG. The method according to embodiment BE, wherein the at least one organic acid comprises at least one of maleic acid or p-toluenesulfonic acid.

BH. The method according to embodiment BE, wherein the at least one organic acid comprises maleic acid.

BJ. The method according to embodiment BC, wherein applying the at least one first coating mixture and applying the at least one second coating mixture occur simultaneously.

BK. The method according to embodiment BC, further comprising drying the at least one first layer or the at least one second layer or both.

EXAMPLES Materials and Methods

All materials used in the following examples are readily available from standard commercial sources, such as Sigma-Aldrich Chemical Co. LLC (Saint Louis) unless otherwise specified. All percentages are by weight unless otherwise indicated. The following additional materials and methods were used.

Zinc nitrate hydrate, Zn(NO₃)₂.xH₂O (Mallinckrodt).

Maleic acid (MA), available from Sigma-Aldrich, having the structure:

p-toluenesulfonic acid (PTSA), available from Fisher/Univar (Martinsville, Va.), having the structure:

CAB 381-20 is a cellulose acetate butyrate resin available from Eastman Chemical Co. (Kingsport, Tenn.). It has a glass transition temperature of 141° C.

CAB 553-0.4 is a cellulose acetate butyrate resin available from Eastman Chemical Co. (Kingsport, Tenn.). It has a glass transition temperature of 136° C.

CYMEL® 303 crosslinker is hexamethoxymethylmelamine, available from Cytec Industries (West Paterson, N.J.).

ESTAR® polyester substrate is available from Eastman Kodak (Rochester, N.Y.).

SLIP-AYD® FS 444 (polysiloxane in dipropylene glycol, Elementis) is a liquid additive for increasing surface slip and mar resistance of water borne and polar solvent borne coatings.

Silver nanowires were prepared according to procedures described in U.S. patent application Ser. 14/043,966, filed Oct. 2, 2013, which is hereby incorporated by reference in its entirety. The typical silver nanowires have diameters ranging from 38 nm to 44 nm and range in length from 17 to 25 μm.

Example 1 Preparation of Silver Nanowire Coating Dispersion

A CAB polymer premix solution was prepared by mixing 15 parts by weight of CAB 381-20 (cellulose acetate butyrate polymer, Eastman Chemical) with 85 parts by weight of n-propyl acetate (Oxea). The resulting CAB polymer premix solution was filtered prior to use.

15.00 parts by weight of the CAB polymer premix solution was combined with 10.00 parts by weight ethyl lactate (>99.8% purity), 40.55 parts by weight of a 1.85% solids dispersion of silver nanowires in isopropanol, and 34.44 parts by weight of n-propyl acetate (Oxea) to form a silver nanowire coating dispersion at 3.00% solids.

The finished silver nanowire coating dispersion was coated on a lab proofer with a 380 LPI plate onto 5 mil ESTAR LS polyester support, and dried at 275° F. for 2 minutes.

Preparation of Topcoat Solutions

A CAB polymer premix solution was prepared by mixing 15 parts by weight of CAB 553-0.4 (cellulose acetate butyrate polymer, Eastman Chemical) into 42.50 parts by weight of denatured ethanol and 42.50 parts by weight methanol (>99% purity). The resulting CAB polymer premix solution was filtered prior to use.

A topcoat masterbatch solution was prepared by adding to 5000 parts by weight of the CAB polymer premix solution 7450 parts by weight denatured ethanol, 4500 parts by weight of 33 wt % CYMEL 303 (hexamethoxymethylmelamine, Cytec) in denatured ethanol, 150 parts by weight of 10 wt % SLIP-AYD FS-444 (polysiloxane in dipropylene glycol ether, Elementis) in denatured ethanol, and 1940 parts by weight of n-butanol (>98% purity). The topcoat masterbatch solution had 12.0% solids.

Topcoat solutions were prepared by adding various loadings of Zn(NO₃)₂.xH₂O, p-toluenesulfonic acid (PTSA), and maleic acid (MA) to aliquots of the masterbatch solution, as shown in Table I.

Preparation of the Coated Films

The topcoat solutions were overcoated on the silver nanowire thin conducting film (AgTCF) described above with a 450 LPI plate. The coatings were then dried in an oven at 275° F. for 3 minutes.

Desktop TCF Stability Test

Surface resistivity was measured for the coatings immediately after coating (initial values) with either a RCHEK RC3175 4-point resistivity or a Delcom 707 non-contact conductance monitor. These TCF samples were then placed on lab desktop under 1500-2000 LUX fluorescence light with the TCF side towards the light for 1, 2, and 4 months. After the test period, the TCF samples were then checked again to record the change in film resistivity.

The stability testing results showed the desktop stability was improved upon addition of either zinc nitrate hydrate (Zn(NO₃)₂.xH₂O), a combination of zinc nitrate hydrate and p-toluenesulfonic acid (PTSA), or a combination of zinc nitrate hydrate and maleic acid (MA). Sample 1-4, having a topcoat layer comprising zinc nitrate hydrate and maleic acid, showed the best results of those examined.

The invention has been described in detail with particular reference to a presently preferred embodiment, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.

TABLE I Surface Surface Surface Resistivity Resistivity Resistivity PTSA Initial Total Change Change Change Zn(NO₃)₂•xH₂0 in MA in Surface light Desktop Desktop Desktop in topcoat topcoat topcoat Resistivity trans. Haze t = 1 month t = 2 months t = 4 months Sample # (wt %) (wt %) (wt %) (ohms/sq) (% T) (%) (% change) (% change) (% change) Com-1-1 none none none 53 90.1 1.88 +42 +136 +323 1-1 0.73 none none 57 89.6 1.66 +17 +53 +332 1-2 1.36 none none 58 89.6 1.62 +15 +48 +180 1-3 1.36 0.13 none 58 89.3 1.57 +7 +44 +168 1-4 1.36 none 0.57 65 89.4 1.67 −13 −13 −12 

What is claimed:
 1. A transparent conductive article comprising: a transparent support; at least one first layer disposed on the transparent support, the at least one first layer comprising a network of silver nanowires dispersed within at least one polymer binder; and at least one zinc salt.
 2. The transparent conductive article according to claim 1, wherein the at least one zinc salt comprises zinc nitrate hydrate.
 3. The transparent conductive article according to claim 1, wherein the at least one first layer further comprises the at least one zinc salt.
 4. The transparent conductive article according to claim 3, wherein the at least one first layer further comprises at least one organic acid.
 5. The transparent conductive article according to claim 4, wherein the at least one organic acid comprises at least one of maleic acid, tetrachlorophthalic acid, trichloroacetic acid, phenylphosphonic acid, p-toluenesulfonic acid, or phthalic acid.
 6. The transparent conductive article according to claim 1, further comprising at least one second layer disposed adjacent to the at least one first layer, the at least one second layer comprising the at least one zinc salt.
 7. The transparent conductive article according to claim 6, wherein that at least one second layer is disposed on the at least one first layer.
 8. The transparent conductive article according to claim 6, wherein the at least one second layer is disposed between the at least one transparent substrate and the at least one first layer.
 9. The transparent conductive article according to claim 6, wherein the at least one first layer further comprises at least one organic acid.
 10. The transparent conductive article according to claim 9, wherein the at least one organic acid comprises at least one of maleic acid, tetrachlorophthalic acid, trichloroacetic acid, phenylphosphonic acid, p-toluenesulfonic acid, or phthalic acid.
 11. The transparent conductive article according to claim 9, wherein the at least one organic acid comprises maleic acid or p-toluenesulfonic acid.
 12. A method comprising: applying at least one first coating mixture onto a transparent support to form at least one first coated layer, the at least one first coating mixture comprising silver nanowires and at least one polymer binder; and, applying at least one second coating mixture onto the at least one first coated layer to form at least one second coated layer, the at least one second coating mixture comprising at least one zinc salt. 